Organization of clathrin coat structures

Clathrin (8S), when purified, polymerizes under low-pH conditions (0.1 M MES, pH 6.0-6.2) into a heterogeneous population of baskets with sedimentation coefficients ranging from 150 to 400 S. Several groups of proteins of molecular masses 180, 110, 100, 50, and 47 kDa (based on sodium dodecyl sulfate gel electrophoresis) present in the isolated coated vesicles are involved in polymerizing clathrin under physiological conditions to a homogeneous population of baskets [Zaremba, S., & Keen, J. H. (1983) J. Cell Biol. 97, 1339; Ahle, S., & Ungewickell, E. (1986) EMBO J. 5, 3143]. We now report that in 0.1 M MES, pH 6.0, where pure clathrin polymerizes by itself, the above proteins (together known as associated proteins or APs) induce polymerization of clathrin into three distinct sizes of baskets with sedimentation coefficients of 150, 220, and 300 S. Low ratios of clathrin to APs give rise to smaller sizes, whereas higher ratios give rise to predominantly the larger sizes. The smaller size baskets (150S) are intermediates in the polymerization of clathrin to larger size baskets (300S) as inferred from the dissociation of larger size baskets into smaller size baskets and the formation of larger size baskets from smaller size baskets upon the addition of pure clathrin.